Cornelius Gurlitt, 80, kept the works by artists including Picasso, Matisse and Renoir inside his home in Munich. They were discovered by chance in 2010, when prosecutors began investigating Mr Gurlitt for tax avoidance, but their existance was only made public at the weekend.

"I think that's as much of a story as the find itself," said Anne Webber, founder and co-chair of the commission for Looted Art in Europe. "It's actually been two and a half years since these paintings were found, and they've been kept completely secret. And there are some very hard questions for the Bavarian government about why they've kept this list secret.

"We need to ask why they haven't published a list of all the paintings that have been found, so that the families who are looking for their paintings - and have been looking for the past 75 years - can find them, and have them returned to them," she added.

Mr Gurlitt's father Hildebrandt was a prominent Munich art dealer, who is thought to have acquired works deemed "degenerate" that had largely been seized from Jewish collectors.

He died in 1956, and his surviving son had apparently kept the collection in his home and is thought to have sold off some of the works as a means of support.

Hildebrandt Gurlitt told the authorities after the war that the art had been destroyed during the Dresden Bombings.

"We represent hundreds of families around the world and we are looking for thousands of paintings, so we and others want to see a list of those paintings immediately," said Mrs Webber. "There is no excuse for sitting on it.

"There is a culture of secrecy in Bavaria, and other parts of Germany - but particularly there. We need a culture of transparency and to return these works as quickly as possible."

She said that many people who dealt with looted Nazi art were interrogated after the war, but claimed that they didn't have anything.

They then continued to sell the works - "trading it, laundering it, particularly in Bavaria," said Mrs Webber.